The present invention relates to an apparatus for converting a voltage signal into a current signal. The invention more particularly relates to a stable, wide-band, bidirectional transconductance amplifier for converting a voltage signal into a current signal which may be advantageously employed in a continuous time domain analog to digital converter.
Transconductance amplifiers are well known in the art. One known transconductance amplifier comprises an operational amplifier with its non-inverting input at the input voltage, its inverting input being connected to the source of a MOS transistor, its output being connected to the gate of the same MOS transistor, and the source of the transistor being connected to ground via a resistor. While such an arrangement is stable over a wide bandwidth, and the output node is at ground, the arrangement is unidirectional as the transistor only allows current to flow in a single direction.
A second transconductance amplifier known in the art an operational amplifier with four matched resistors. The input voltage is connected to the inverting input via a first resistor. The non-inverting input is connected to ground via a second resistor. A third resistor bridges the output and the inverting input of the op amp. A sense resistor (unmatched) is located in series between the op amp output and the output of the transconductance amplifier, and a fourth matched resistor bridges the transconductance amplifier output and the non-inverting input to the operational amplifier. While the provided output current is equal to the input voltage divided by the resistance of the fourth resistor and is thus bidirectional, the arrangement is unstable and is useful only over a limited bandwidth.